Officials in Ukraine say they have detained four Ukrainian citizens in connection with an explosion in the eastern city of Kharkiv that left two dead and 11 injured.

Police say the people killed in the February 22 explosion were a police officer and a civilian. Four of the wounded were also police officers.

Oleksandr Turchynov, head of Ukraine's National Security and Defense Council, said a "counterterrorism operation" has been launched in Kharkiv in response to the incident.

The bomb struck a peace march that was being held to mark the first anniversary of the overthrow of former President Viktor Yanukovych.

An official with Ukraine's SBU state security service was quoted as saying the four people who were detained "underwent instruction and received weapons" in the Russian city of Belgorod.

Kyiv and its Western allies have repeatedly accused Moscow of direct aid to separatist fighters, including through the supply of weapons, soldiers, instruction, and other equipment.

People carry portraits of local residents killed in fighting with pro-Russian separatists during a commemoration rally in Kharkiv on February 22.

A Kharkiv prosecutor was quoted by Interfax as saying the bomb was "filled with shrapnel." A local SBU official said a similar weapon had been located and disarmed in Kharkiv on February 19.

Kharkiv is located more than 200 kilometers from the front line.

"Today is memorial Sunday, but on this day terrorist scum revealed its predatory nature," Ukrainian President Petro Poroshenko said on Facebook.

Poroshenko led the march in Kyiv, joined by thousands of other participants, as well as the presidents of Poland, Lithuania, Germany, Georgia, and the European Union.

Ukraine's parliament voted to oust Yanukovych on February 22, 2014, after months of protests in Kyiv.

A month later, Russia annexed Ukraine's Crimea region.

Armed rebels then took over parts of two regions bordering Russia -- Luhansk and Donetsk -- triggering a conflict that has killed more than 5,600 people.

WATCH: Protesters gathered in the northeastern Ukrainian city of Sumy on February 22 to burn an effigy of Russian President Vladimir Putin. The protest took place as demonstrations were under way in other Ukrainian cities to mark the first anniversary of the overthrow of former Ukrainian President Viktor Yanukovych. (RFE/RL's Ukrainian Service)

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A fresh peace plan including a cease-fire and a weapons pullback was signed on February 12.

The Ukrainian military says it will not start withdrawing heavy weapons from the front line in the conflict with Russian-backed separatists because the rebels have not completely ceased fire.

Under an agreement brokered by the leaders of Germany and France, government forces and rebels were to cease fire on February 15 and begin pulling back heavy weapons no less than two days later, creating a security zone at least 70 kilometers wide.

Ukrainian military spokesman Anatoly Stelmakh said on February 23 that while hostilities have lessened in recent days after separatist forces took the strategic town of Debaltseve, rebels shelled government troops twice overnight.

Another spokesman, Vladyslav Seleznyov, said in a televised briefing that because "the positions of Ukrainian servicemen continue to be shelled, there cannot yet be any talk of pulling back weapons."

Stelmakh said Ukraine would begin the withdrawal when "the enemy stops firing on our positions."

Ukrainian military spokesman Andriy Lysenko said the rebels were pressing on with attacks on government forces nearby the city of Mariupol on the Sea of Azov.

With 500,000 people, Mariupol is the biggest government-held city in the regions of Donetsk and Luhansk.

Its capture could allow the rebels to create a land bridge from Russia to Crimea.

Lysenko said a military train carrying 60 armored vehicles including tanks had arrived in the town of Amvrosiivka from Russia on February 21.

A convoy of military equipment had later crossed the border near Novoazovsk, east of Mariupol.

He said fighting was in progress at the village of Shyrokyne, east of Mariupol.

"Our soldiers are holding their positions," Lysenko said, adding that there had been a total of 44 attacks by separatists across the conflict zone in the past 24 hours.